Dr. Joshua Ryan Smith studies energy conversion devices. In his most recent paper he simulated a thermionic engine that is made from currently available materials and determined the conditions for which it achieves 20% efficiency.

Efficient energy production is a perennial challenge facing the Army. Recent work at the U.S. Army Research Laboratory's Adelphi Laboratory Center may provide a means to meet that challenge in the form of a thermionic engine. Calculations show this device can produce an output of one kilowatt at 20 percent or higher efficiency, and would be roughly the length and width of a sheet of paper and made from available materials.

Dr. Joshua Smith has simulated the performance of a novel vacuum thermionic engine based on currently available materials in a paper entitled, "Increasing the Efficiency of a Thermionic Engine using a Negative Electron Affinity Collector," which was recently published in the Journal of Applied Physics.

Smith, an Oak Ridge Associated Universities Senior Researcher in the Power Components Branch of the Sensors and Electrons Devices Directorate, explained that the primary focus of his paper was to show the plausibility of achieving 20 percent or greater efficiency in a thermionic engine.

Thermionic engines, sometimes called thermionic energy conversion devices or TECs, are similar to vacuum tubes and were studied extensively during the Cold War by scientists both in the United States and the former Soviet Union.

Several prototypes were built, but scientists at the time faced numerous challenges as a result of rudimentary materials selection and fabrication techniques. "The thermionic engine converts heat directly to electricity and is based on a phenomenon called thermionic emission, which is basically the emission of electrons from a heated material," Smith says. These devices can be very efficient because there's no direct conduction of heat across the device, but past scientists had to contend with a phenomenon known as the negative space charge effect. Since electrons traverse the device in a vacuum, a negative charge builds up within the device and blocks some of the electrons from crossing the device.

"I simulated a TEC with a negative electron affinity collector made of diamond, and a scandate emitter. I showed that the collector's negative electron affinity reduces the space charge effect and allows more current to flow through the device, increasing the output power and efficiency," added Smith.

Smith's design operates at a temperature of 1,000 Kelvin which is around 700 degrees Celsius. Thermionic engines offer promise for use in a variety of military and commercial applications. One possible application could be to mount a TEC device to a vehicle exhaust system, and, in effect, recycle the engine exhaust heat into usable energy to charge a battery or power an auxiliary device on the vehicle. Smith noted, "Because there are no moving parts (in a thermionic engine), there will be a significant reduction in noise and size, therefore the reduction in weight will increase overall efficiency." Lighter and more reliable power sources also serve to reduce the logistical burden for both Soldiers and vehicles.

While this technology is not yet mature enough for widespread applications, according to Smith, five to ten years seems like a realistic timeframe for this technology to be sufficiently mature for military use. "Scientifically, the next step is to develop and test a prototype of the device. Beyond that, it turns into engineering matter in terms of how to scale it up in production."

Not only is this success story an important technical achievement for Smith and ARL, it also serves to demonstrate the importance of leveraging the expertise of ARL's visiting scientists and the benefits that can result in general from successful collaborations and partnerships with academia and others.

Smith's research was sponsored by the U.S. Army Research Laboratory. His paper was published open-access, as was his code for this simulation. To view his paper published in the Journal of Applied Physics in its entirety please visit http://dx.doi.org/10.1063/1.4826202. The code can be found at http://github.com/jrsmith3/tec.